The conventional AlGaInP LED, as shown in FIG. 4, has a double heterostructure (DH), which is consisted of an n-type (Al.sub.x Ga.sub.1-x).sub.0.5 In.sub.0.5 P lower cladding layer 4 with an Al dosage of about 70%.about.100%, formed on an n-type GaAs substrate 3, an (Al.sub.x Ga.sub.1-x)0.5In.sub.0.5 P active layer 5, a p-type (Al.sub.x Ga.sub.1-x).sub.0.5 In.sub.0.5 P upper cladding layer 6 with an Al dosage 70%.about.100% and a p-type high energy gap GaP or AlGaAs current spreading layer 7.
There are some conventional LED technologies have been disclosed in order to avoid the absorption of light by the substrate. However, these conventional technologies still have some disadvantages and limitations. For example, Sugawara et al. disclosed a method, which has been published in Appl. Phys Lett. Vol. 61, 1775-1777 (1992), that adding a distributed bragg reflector (DBR) layer on the GaAs substrate so as to reflect the light ejected to the GaAs substrate and to decrease the light absorbed by the GaAs substrate. However, because the DBR layer only can effectively reflect the light approximated to verticality ejected to the GaAs substrate, so that the efficiency is not very great.
Kish et al. disclosed a wafer-bonded transparent-substrate (TS) (Al.sub.x Ga.sub.1-x).sub.0.5 In.sub.0.5 P/GaP light emitting diode [Appl. Phys Lett. Vol. 64, No. 21, 2839 (1994); Very high-efficiency semiconductor wafer-bonded transparent-substrate (Al.sub.x Ga.sub.1-x).sub.0.5 In.sub.0.5 P/GaP]. This TS AlGaInP LED was fabricated by growing a very thick (about 50 .mu.m) p-type GaP window layer using hydride vapor phase epitaxy (HVPE) together at a temperature above the eutectic point of AuSn solder. After bonding, the n-type GaAs substrate was selectively removed using conventional chemical etching techniques. The exposed n-type layers subsequently wafer-bonded to 8-10 mil thick n-type GaP substrate. The resulting TS AlGaInP LED exhibit a two fold improvement in light output compared to absorbing substrate (AS) AlGaInP LED. However, the fabrication process of TS AlGaInP LED is too complicated. Therefore, it is difficult to manufacture these TS AlGaInP LEDs in high yield and low cost.
Horng et al. reported a mirror-substrate (MS) AlGaInP/metal/SiO.sub.2 /Si LED fabricated by wafer-fused technology [Appl. Phys Lett. Vol. 75, No. 20, 3054 (1999); AlGaInP light-emitting diodes with mirror substrates fabricated by wafer bonding]. They used the AuBe/Au as the adhesive to bond the Si substrate and LED epilayers. However, the luminous intensity of these MS AlGaInP LEDs is about 90 mcd with 20 mA injection current and is still 40% lower than the luminous intensity of TS AlGaInP LED. Besides, both p-electrode and n-electrode are formed on the same side, so that the chip size can not be decreased. Therefore, the chip size is larger than conventional LED chip that has p-electrode on one side and n-electrode on the other side. Thus, this type of LED chip is difficult to satisfy a case of a package size compatible with the trend toward miniaturization.